I’d say the biggest mistake is only doing rear delts at the end of a workout , with light weights and random reps. Honestly, I used to do that and nothing really stuck. A few exercises that have worked really well for me are:
Reverse pec deck: probably my favorite; it feels pretty easy to keep tension on the rear delts the whole time.
Cable rear delt flyes: the cables give constant tension, so you can actually focus on the muscle more than on, like, yanking or swinging.
Face pulls: great for rear delts and also for shoulder health overall , if you pull toward your upper face and keep your elbows high.
Chest-supported rear delt dumbbell flys: supporting your chest helps take momentum out of the movement.
Wide-grip rows ( elbows flared ): this one hits the rear delts nicely, along with the upper back too.
The biggest difference for me came from using controlled reps, pausing for a second at the peak contraction , and training the rear delts 2–3 times a week instead of just once. They recover pretty well, so a little extra volume usually pays off.